Surface treatment of zinc/aluminium alloys

ABSTRACT

IN A METHOD FOR THE SURFACE TREATMENT OF AN ALLOY CONSISTING PREDOMINANTLY OF ZINC AND ALUMINUM, AND CONTAINING 60-85% BY WEIGHT OF ZINC BY IMMERSING A SURFACE OF THE ALLOY IN DILUTE AQUEOUS SULPHURIC ACID, COMPRISING SUBJECTING THE ALLOY TO ANODIC OXIDATION AT A CURRENT DENSITY OF FROM 50 TO 200 MILLIAMPERES PER SQUARE CENTIMETRE OF SURFACE. THE ALLOY MAY INCLUDE UP TO 0.5% BY WEIGHT OF COPPER OR MAGNESIUM AS A THIRD METAL.

United States Patent US. Cl. 204-32 R Claims ABSTRACT OF THE DISCLOSUREIn a method for the surface treatment of an alloy consistingpredominantly of zinc and aluminium, and containing 60-85% by weight ofzinc by immersing a surface of the alloy in dilute aqueous sulphuricacid, comprising subjecting the alloy to anodic oxidation at a currentdensity of from 50 to 200 milliamperes per square centimetre of surface.The alloy may include up to 0.5% by weight of copper or magnesium as athird metal.

This invention relates to the surface treatment of zinc/ aluminiumalloys to produce thereon a protective and/ or decorative surfacefinish.

It is well known that zinc can be anodically oxidised, usingconventional DC techniques and electrolytes based on sodium hydroxide orsodium carbonate, to give a white film at high current densities and ablack film at low current densities. It is also known that alloys ofzinc containing 4 to 12% aluminium can be similarly anodised in similarelectrolytes again to give white or black films. However, with zinc orthe low-aluminium alloys mentioned these films produced have not beenfound useful for practical application since they comprise mainly zincoxide.

.It is standard practice in the aluminium industry to anodise highpurity or even commercial grade aluminium in 15 to 25% (w./v.) solutionsof sulphuric acid to produce films of aluminium oxide which can confercorrosion resistance or decorative appearance to articles made of themetal so treated. It is also known that aluminium alloys containing afew percent of zinc by Weight can be anodised in a 15% solution byweight of sulphuric acid to produce aluminium oxide films containingzinc values in approximately the alloying proportions. It has been foundimpossible to anodise zinc in any sulphuric acid solution or a 55/45zinc/aluminium alloy in 4 N sulphuric acid solution at a current densityof 10 ma./ sq.

The invention consists in a method for the surface treatment of an alloyconsisting predominantly of zinc and aluminium, and contaiining 60-85%by weight of zinc, comprising immersing a surface of the alloy in diluteaqueous sulphuric acid and subjecting it to anodic oxidation at acurrent density of from 50 to 200 milliamperes per square centimetre ofthe surface.

It will be apparent that the current density employed in the presentmethod is considerably higher than that used in conventional aluminiumanodizing operations, which is typically 10-20 milliamperes per squarecentimetre.

Thus zinc/aluminium alloys containing 60-85% by weight of zinc may beanodically oxidised in aqueous solutions of sulphuric acid to producefilms which are basically aluminium oxide with only a small content ofzinc, probably as oxide, of about 510% by weight of the anodised film.Moreover an accelerated corrosion test has indicated that this filmconfers corrosion protection on the alloy and that sealing treatment ofthe type 3,729,391 Patented Apr. 24, 1973 "Ice applied to anodised filmson aluminium also has a beneficial effect on these films.

The alloy is usually either an alloy of high-purity zinc, orcommercial-purity zinc (i.e. containing only incidental impurities, aswell known to the man skilled in the art) with correspondinglyhigh-purity or commercial-purity aluminium. While the alloy can have acomposition containing up to by weight of zinc, i.e. as little as 15% byweight of aluminium, in practice it is most preferred to usecompositions containing 60-80%, or even 60- 70% of zinc by weight.Although the alloys to be treated in the present method consistpredominantly of zinc and aluminium it is possible to include minoramounts of a ternary metal as deliberate additions, e.g. up to 0.5% ofcopper or magnesium may be added.

The microstructure of the alloy is usually that produced by casting andmechanical working, and will generally comprise alternate regions, e.g.of approximately 1,u. diameter (i.e. 10- metres) of zinc-rich andaluminium-rich phases.

Zinc may be removed from the surface of the alloy prior to anodizinge.g. by evaporation in vacuo. However, it is a valuable feature of thepresent method that by suitable control of the anodizing conditions thepreferential leaching of a zinc phase and the anodizing of analuminium-rich phase may be carried out simultaneously in the same bathof sulphuric acid.

The sulphuric acid concentration is preferably between 1% and 40%(W./v.) and the most preferred range is between 10% and 20% (w./v.) H 80in water. The anodizing is preferably carried out at a temperaturewithin the range 030 C. Above 30 C. the anodized layer becomes lessadherent.

It is preferred to anodise in the constant current mode, althoughpossible to operate in the constant voltage mode.

It is further envisaged to carry out an optional sealing treatment onthe anodised films produced. Sealing can be effected in boilingdistilled water.

The invention will be further described with reference to the followingexamples which describe anodising treatments carried out oncommercial-purity and high-purity zinc/aluminium alloys. Six samples ofalloy were treated as summarised below.

Current densities Sample Description used (ma/em?) A Commercial purityzinc and 30% 150, 100, 50 and 25.

aluminium.

B Commercial purity zinc and 40% 150, 100, 50 and 25.

aluminium.

C High purity zinc and 30% aluminium. 150, 100, 50 and 25.

D High purity zinc and 40% alumininrm. 150, 100, 50 and 25.

E Commercial purity zinc and 15% 150, and 50.

aluminium.

F Commercial purity zinc and 22% 150, 100 and 50.

aluminium.

(a) ALLOYS A AND C At ma./cm. voltage/time curves for bothcommerical-purity and high-purity alloys (A and C) initially exhibited aregion of very low voltage (about l-2 volts) which lasted for about 30seconds. This was followed by a sharply rising voltage/time curve regionwhich was less steep in the case of the high-purity alloy C. At about 8v. this subsided into a region in which the voltage rose more gradually,then the gradient of this region also increased giving a peak of 30 v.after 7 minutes for both alloys. Both alloys then exhibited a furthershort but steep voltage increase which terminated in a region of gentlevoltage fluctuation at about 34 v. for commercial-purity alloy A andabout 35 v. in the case of high-purity alloy C.

At 100 ma./cm. similar features in the voltage/time curve were observedbut the various regions took longer to become evident. A peak of 30 v.was reached after 14 minutes for the high-purity alloy C.

At 50 ma./cm. the various regions of the voltage/time curve appeared asbefore although the final peak took up to 2 hours to form.

(in) ALLOYS B AND C At 150 ma./cm. alloys -B and D exhibited a sharpvoltage increase starting as soon as the current was switched on. Thisregion terminated at 15 v. for both alloys although this Was reachedafter 10 seconds by the highpurity alloy D and 40 seconds by thecommercial-purity alloy B. There was no initial low voltage region foralloys B and D as there has been for the zinc-30% aluminium alloys A andC. A region of more gradual voltage increase followed peaking at 49 v.after 4.5 minutes for the commercial-purity alloy and 35 v. after 2.5minutes for the high-purity alloy. This was followed for both alloys Band D by a gently fluctuating voltage region, similar to that for thezinc-30% aluminium alloys A and C though with larger fluctuations in thecase of the high-purity alloy D.

At 100 ma./cm. similar behaviour occurred. The peaks were at 49 v. after7 minutes for commercial-purity alloy B and 37 v. after minutes for thehigh purity alloy D. At 50 ma./cm. both alloys B and D exhibited aninitial period of low voltage (about 1 v.) which terminated after 1.5and 0.5 minutes for the commercial-purity alloy B and high purity alloyD respectively. Commercial-purity alloy B then exhiibted a voltageplateau of about 3 v. which was still evident after 12 minutes.High-purity alloy D exhibited a sharp voltage increase reaching v. after3 minutes, easing off to reach 14 v. after 8 minutes, and rising steeplyagain to give a peak of 30 v. after 12.5 minutes, before entering regionof gentle voltage fluctuation.

(c) ALLOY B At 150 ma./cm. alloy exhibited a sharp voltage increase to 5v. after 7.5 minutes after a region of very low voltage 12 V.) lasting 5minutes. This voltage was maintained until a further sharp rise startedat 12.5 minutes to give a peak of 40 v. after 17.5 minutes. The voltagethen dropped slowly to v. after 26 minutes, increased again slowly to 40v. at 40 minutes and dropped once again to 25 v. at 42 minutes.Thereafter the voltage gently fluctuated about this last value up to 60minutes.

At 100 ma./cm. the voltage began to rise slowly after a region of verylow voltage (12 V.) lasting 5 minutes, and then more steeply to give apeak of 50 v. after 25 minutes. The voltage then dropped slowly to give15 v. after 34 minutes, and then rose slowly to give 35 v. after 60minutes.

At 50 ma./cm. a very gradual rise in the voltage to about 3 v. after 60minutes occurred.

(d) ALLOY F At 150 ma./cm. alloy F exhibited a region of very lowvoltage (1-2 V.) lasting 2 minutes followed by a voltage rise to aplateau of 8 v. after 3 minutes. This value Was maintained until a sharprise started after 7.5 minutes to give 30 v. after 12.5 minutes. Thevoltage then rose more gradually to 45 v. after 22.5 minutes. At thisstage a voltage surge to over 100 v. occurred at about 25 minutes. Thevoltage then dropped sharply to 30 v. and thereafter fluctuated gentlyabout this value.

At 100 ma./cm. the voltage rose to a platen of 4 v. after 6 minutesafter an initial low voltage region of about 12 v. lasting 5 minutes.After about 16 minutes a sharp rise occurred reaching 30 v. after 22.5minutes. The voltage then rose more gradually to about 45 v. after 35minutes. It then dropped gradually to about 40 v. after 47.5 minutes andthen more sharply to 25 v. after 50 minutes. Thereafter the voltageremained approximately at this value.

At 50 ma./cm. a very gradual rise in voltage to about 3 v. after 60minutes occurred.

Tests were also carried out on commercial purity zinc/ aluminium alloyscontaining small amounts of magnesium and copper. Two alloy samples weretreated as summarised below.

Current densities used Sample Description (ma/cm?) G Commercial purityzinc, 22% aluminium, 150, and 50.

and 0.01% magnesium.

H Commercial purity zine, 22% aluminium 150, 100 and 50.

and 0.15% copper.

(e) ALLOY G At ma./cm. alloy G exhibited a voltage rise to a plateau of4 v. at 4 minutes after an initial low voltage region (12 V.) lasting 3minutes. At 10 minutes the voltage rose sharply to give 30 v. after 17.5minutes. This value was maintained until 32.5 minutes. The voltage thendropped gradually to a minimum of 22 v. after 42.5 minutes and then roseagain to 30 v. after 60 minutes.

At 100 ma./cm. the voltage rose gradually to 3 v. after 20 minutes. Thisrise steepened to 30 v. after 35 minutes. The voltage then dipped to 28v. after 42.5 minutes and rose again to 37 v. after 60 minutes.

At 50 ma/em. the voltage rise was very slow giving 2-3 v. after 60minutes.

(f) ALLOY H At 150 ma./ :m. alloy H exhibited a voltage rise to 25 v.after 12.5 minutes after an initial region of very low voltage (l2 v.)lasting 2 minutes. Thereafter the voltage fluctuated between 25 v. and35 v. until 60 minutes (the duration of the investigation) had elapsed.

At 100 ma./cm. the voltage rose to 4 v. after 6 minutes after an initiallow voltage region of 1-2 v. lasting 5 minutes. The voltage then roseincreasingly sharply to give 26 v. after 25 minutes. It then rosegradually to 35 v. after 52.5 minutes and then dropped to 25 v. after 60minutes.

At 50 ma./cm. the voltage rose very slowly giving about 3 v. after 60minutes.

There is given below corrosion test results carried out on anodisedcommercial grade 60:40 zinc/ aluminium alloy.

CORROSION TEST DATA Current density Specimen Treatment time N 0. (ma./cm.

Terminal voltage (V.) (min.)

5 As 4 and sealed as before 100 35 9. 5

As 6 and sealed as before As 8 and sealed as before Test-results (usingthe salt-fog test):

120 h. salt fog 200 h. salt fog Rating Percent Percent Specimen No.(White rust) staining Rating staining Rating is on a scale 10-perfect to0, and staining (i.e. loss of uniformity of coating colour) is on ascale 0 (perfect) to 100%. These give an order of merit of 120 h., 200h., Overa 1, Best No. 3 No. 3 No. 3 Best 3 2 }Little difference.

214,6 2 2' 4, }Little difference.

1 Worst.

SPECIMEN FILM THICKNESS-TIME DATA The figures in the table below referto the thickness of the anodic film (in microns) formed during anodisingof Zn-22% Al at 100 ma./cm. in w./v. sulphuric acid at C. for varioustimes.

Time (min): Film thickness (micron) 10 The figures tabled areapproximate.

We claim:

1. In a method for the surface treatment of an alloy consistingpredominantly of Zinc and aluminium, by immersing a surface of the alloyin dilute aqueous sulphuric acid, the improvement in combinationtherewith comprising subjecting the alloy containing to by weight ofzinc to anodic oxidation at a current density of from 50 to 200milliamperes per square centimetre of the surface.

2. A method as claimed in claim 1 wherein the alloy includes up to 0.5%by Weight of a th rd metal selected from the group consisting of copperand magnesium.

3. A method as claimed in claim 1 wherein zinc is removed from thesurface of the alloy prior to anodizing by evaporation in vacuo.

4. A method as claimed in claim 1 wherein leaching of the zinc phase andthe anodizing of the resulting aluminium-rich phase are carried outsimultaneously in a common sulphuric acid bath.

5. A method as claimed in claim 1 wherein the sulphuric acidconcentration is between 1% and 40% (w./v.) in water.

6. A method as claimed in claim 5 wherein the sulphuric acidconcentration is between 10% and 20% (w./v.) in water.

7. A method as claimed in claim 1 wherein the anodizing is carried outat a temperature between 0 and 30 C.

8. A method as claimed in claim 1 further comprising sealing theanodized film produced in boiling distilled water.

References Cited UNITED STATES PATENTS 3,335,074 8/1967 Wright et a1.20456 R JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant ExaminerU.S. Cl. X.R. 20435, 56, 58

